Many hydrocarbon producing wells are now hydraulically fractured or fracked shortly after drilling. In one technique, after the production casing string is run and cemented, the operator will perforate the casing string and perform the fracturing operation. A frac adapter will mount on the upper end of the wellhead housing. The operator pumps high pressure frac fluid down the casing string, which flows out the perforations to form cracks or fissures in the earth formation. Afterward, the frac tree is removed and the wellhead configured for production.
The production fluid, often a mixture of gas, oil and water, will normally not have a very high pressure at the wellhead. During the fracturing process, the pressure in the wellhead will be high, often more than 5000 psi and in some cases more than 10,000 psi. To reduce the cost of the wellhead, the operator will typically install a wellhead assembly that has a pressure rating much lower than the expected fracturing pressure. For example, a production wellhead housing along with associated valves may have only a 5000 pressure rating.
A variety of devices are employable to avoid damage to the production wellhead equipment if the frac pressure exceeds the pressure rating. Generally, these devices insert into the bore of the production wellhead housing to protect the valves and production wellhead housing.